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Ag/C, O
2
KOH
O
Ph
OH
Ph
O
S
DMSO
Ph
4
5
XV
Ag/C, O
2
O
O
S
O
Ph
S
Ph
Ph
Ph
O
O
O
Ph
65a
XVI
Ph
O
Scheme 7.17 Proposed mechanism of the formation of styryl ethers from benzyl
alcohols.
and styryl sulfoxide as the key intermediate. The authors proposed the
mechanism shown in Scheme 7.17. Silver nanoparticles first catalyzed
the aerobic oxidation of benzyl alcohol 4 to benzaldehyde 5. The addition of
the anion of DMSO followed by elimination of water produced the
styryl sulfoxide intermediate XV. The latter was oxidized by the silver nano-
particles to afford the corresponding styryl sulfone XVI, which underwent an
addition-elimination with benzyl alkoxide to produce the observed styryl
benzyl ether 65a.
Iron also mediated cascade redox reactions for the synthesis of N-het-
erocyclic structures possessing interesting biological activities. Pereira and
Thi´ry reported the synthesis of pyrrolo[1,2-a]quinoxalines 68 and pyr-
rolo[1,2-a]pyrazines 69 from alcohols and 2-nitrophenylpyrrole 66 and
3-nitro-2-pyrrolopyridine 67, respectively, using a stoichiometric mixture of
iron powder and hydrochloric acid (Scheme 7.18).
41
In this reaction, iron played a dual redox role: it not only promoted the
aerobic oxidation of the alcohol moiety, but also served to reduce the nitro
group, forming the ammonium intermediate XVII under strongly acidic
conditions (Scheme 7.19). Upon formation of the iminium XVIII from the
aldehyde and the ammonium components, intramolecular cyclization took
place, followed by oxidation (probably mediated by iron species under an air
atmosphere), leading to the N-heterocyclic product 68 or 69.
Exploiting the modular combination of transition metal catalysis and
organocatalysis, Kudo and co-workers designed an oxidation-asymmetric
aldol multicatalytic reaction (Equation 7.15).
42
In a model reaction,
2-nitrobenzyl alcohol 70 was subjected to aerobic oxidation in presence of
poly(ethylene glycol)-polystyrene-TEMPO resin (PEG-PS-TEMPO) along
with a CuCl-bpy complex. Without purification, the aldehyde intermediate
underwent a PEG-PS-proline-catalyzed asymmetric aldol reaction with
acetone to form the b-hydroxy ketone 71 in 73% yield with good enantios-
electivity (85% ee). Importantly, the introduction of triglycerine between the
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